The present invention relates to video communications, and more particularly to methods, apparatus, and systems for adjusting an automatic gain control (AGC) for a tuner to reduce intermodulation distortion (IMD) and noise in a television picture or the like.
In a “settop box” for cable television reception, or any other device that demodulates RF modulated video to baseband video, the tuner is generally considered to be the most critical section in terms of noise and intermodulation distortion (IMD). Noise causes random fluctuations in the picture, often referred to as “snow.” For purposes of simplicity, IMD can be described as causing diagonal lines in the picture. The visual effects of IMD are actually much more involved than this simple characterization, and have been the subject of much research.
To minimize noise, and thus maximize signal-to-noise ratio (SNR), the input level to the tuner should be maximized. To minimize IMD, the input level to the tuner, post AGC, should be minimized because IMD occurs in the tuner front end active components, and is proportional to the total input power (i.e., the input power of all the channels combined). Therefore a balance is set for acceptable noise and IMD. In this terminology, the “input level to the tuner” actually refers to the tuner level at the first active device thereof, typically the up-converter.
FIG. 1 shows the resulting noise and IMD with increasing input channel power, with the tuner set at full gain (no tuner AGC attenuation). As can be seen from this figure, a balance is needed between SNR and IMD. In practice, this balance is controlled by an automatic gain control in the tuner front end.
FIG. 2 is a simplified block diagram of a typical prior art television tuner 14. The power level to be optimized is at the input to the up-converter 18, and is controlled by the AGC attenuation function 16. An AGC control line 20 provides a voltage level that controls the amount of attenuation applied between the RF input 22 and the up-converter 18. For low level input power conditions, the tuner is set to full gain, i.e. no AGC attenuation. For high level input power conditions, AGC attenuation is applied in an amount proportional to the power level.
The RF input power point at which AGC attenuation is first applied is termed the “Attack Point.” For input levels above the attack point, the up-converter will see a fixed power level equal to the attack point. For input levels below the attack point, the up-converter will see the RF input level, since the AGC attenuation is set to no attenuation.
FIG. 3 shows the full video processing path from RF input 22 to demodulated digitized video 24. Digitized and separated video 24 (e.g., in CCIR656 Y/Cr/Cb format) is provided by the video decoder 26, which contains an analog to digital converter (A/D) and separation blocks.
The attack point setting 28 at the video demodulator 30 establishes a level limit in the demodulator, thereby limiting the signal level into the tuner's up-converter at the desired attack point of the tuner. The attack point is a fixed value that is set for each unit when manufactured, to compensate for the overall tuner gain (post AGC attenuation), IF amplifier gain, and demodulator unit to unit variations.
There are various drawbacks to the fixed AGC control practiced in the prior art as described above. Although the fixed AGC attack point control has worked for many years in analog and digital settop boxes, it is not optimal in providing the best quality picture. With the advent of affordable large screen TVs, the best possible picture quality is essential. Among the drawbacks to prior art AGC implementations are the following:    1. The attack point is set for one RF channel. Since the overall tuner gain varies with respect to channel frequency, the actual attack point changes versus frequency. Therefore the desired balance between noise and IMD is not maintained. The actual attack point will be too high for low tuner gain channels, and too low for high tuner gain channels.    2. The actual attack point moves versus temperature, due to the tuner gain and IF path gain changing with respect to temperature.    3. If the particular tuned channel has a low RF level compared to the rest of the cable television channels, the tuner up-converter will see too much power (due to the high power levels of the non-tuned channels) and IMD will be too high.    4. If the particular tuned channel has a high RF level compared to the rest of the cable channels, the tuner up-converter power level will not be as high as it could be for best video SNR.    5. If a particular tuner has a very good noise figure overall, or a good noise figure on certain channels, lowering the attack point would yield better IMD performance while still providing good video SNR. Conversely, a tuner with worse noise figure characteristics would need a higher attack point to maintain good video SNR.    6. The attack point is chosen for the worst case cable plant loading, i.e., all analog channels. With digital channels now occupying much of the channel space, the attack point could be raised, since the digital channels are at a lower power level and distortion effects are not as visible. However the attack point is only set once, at factory build time, where the end user's cable loading characteristics is unknown and dynamic.
Accordingly, it would be advantageous to provide a dynamically controlled AGC attack point for television tuners and the like. The present invention provides methods and apparatus for implementing such a dynamic control, thereby improving both SNR and IMD in a television picture or other video display derived from an RF input signal.